It is already known that base stations in mobile radio systems emit a multicarrier transmission signal in the downlink (that is to say on the radio path from the base station to the mobile radio stations). The multicarrier transmission signal includes the signal streams which are intended for the various mobile stations. The characteristic of the multicarrier transmission signal is that two or more carriers (typically adjacent frequency bands) are used, with the signal streams split between them. Multicarrier signal transmission is used in the downlink in many mobile radio systems, for example GSM (Global System for Mobile Communications). Multicarrier transmission signals can also occur in the downlink in CDMA (Code Division Multiple Access) third-generation mobile radio systems, which use spread coding of the individual signal streams for subscriber separation. By way of example, the WCDMA (Wideband Code Division Multiple Access) signal in the UMTS (Universal Mobile Telecommunications System) mobile radio system can optionally be transmitted using the UTRA FDD (Universal Terrestrial Radio Access Frequency Division Duplex) mode in the downlink via two or more frequency channels, each with a channel bandwidth of 5 MHz.
The definition of the WCDMA downlink signal in the UMTS Standard is specified in the 3GPP Standard TS 25.213 v5.3.0 (2003-03) Spreading and Modulation (FDD).
Downlink signals which are emitted from base stations, that is to say including multicarrier transmission signals, typically have to comply with specific quality requirements. For UMTS, these quality requirements relating to the UTRA FDD mode are specified in the 3GPP Standards TS 25.104 v6.2.0 (2003-06) Base Station (BS) radio transmission and reception (FDD) and TS 25.141 v6.2.0 (2003-06) Base Station (BS) conformance testing (FDD). The requirements defined in the Standards state that, when using specific reference signals (which are defined in the Standards), measurements of various quality variables must be carried out, in which case the measured values must be within a tolerance band that is specified in the Standard. Five different test modes (test modes 1 to 5) with different reference signals and the quality variables (EVM (Error Vector Magnitude), PCDE (Peak Code Domain Error) and ACLR (Adjacent Channel Leakage power Ratio) are defined in the abovementioned Standards.
A multicarrier transmission signal which is formed in a base station is composed of a large number of signal streams, with each signal stream being associated with one specific carrier. As will be explained in more detail in the following text, this multicarrier transmission signal also has to be raised to the desired radio-frequency transmission band and has to be amplified by means of a power amplifier before it is emitted via the antenna. During the process, the difficulty arises that the multicarrier transmission signal has a wide dynamic range, owing to the large number of carriers and signal streams associated with them. The power amplifier is that component of the base station which is most affected by the wide dynamic range. This is because the power amplifier has to have a linear response over its entire input dynamic range. If a power amplifier with an excessively narrow linear input dynamic range is chosen, the quality requirements which are specified in the abovementioned Standards relating to the emitted signal cannot be complied with. This means that:                the power amplifier in a base station for transmission of a multicarrier transmission signal must have an “overdesigned” linear input dynamic range in order to comply with the stringent dynamic range requirements,        a more complex cooling system is required for an overdesigned power amplifier,        the requirements for the power supply system for the power amplifier are more stringent, and        the electrical power consumption of the power amplifier is greater.        
All of the items mentioned increase the costs for the network operator, and in this context it should be noted that the power amplifier is typically the most expensive component in a base station.
One possible way to use lower-cost power amplifiers with a narrower linear input dynamic range is to provide a dedicated power amplifier for the signal streams of each carrier. In this case, the individual power amplifiers do not require such a wide linear input range. The disadvantage of this procedure is, however, that a number of power amplifiers (one power amplifier per carrier) are required, thus cancelling out the cost advantage.
Another possibility is to reduce the dynamic range of the multicarrier transmission signal. In this case, the (single) power amplifier may have a narrower linear input dynamic range.
A first known technique for reducing the dynamic range of the multicarrier transmission signal is to superimpose pulses on the multicarrier transmission signal in the passband or in the low-frequency band, which compensate for signal peaks in the multicarrier transmission signal, so that these are brought within a desired threshold value. This technique is proposed in the documents “Multi-Carrier WCDMA Basestation Design Considerations—Amplifier Linearization and Crest Factor Control”—White Paper—Andrew Wright—PMC Sierra—Aug. 1, 2002, “Reducing the Peak-to-Average Power Ratio in OFDM Radio Transmission Systems”,—T. May, H. Rohling, Proc. IEEE VTC '98, Phoenix May 1998, and “Additive Algorithm for Reduction of Crest factor”—N. Hentati, M. Schrader—5th International OFDM Workshop 2000, Hamburg.
A further technique for reducing the dynamic range of a multicarrier transmission signal is specified in the document “Effect of Clipping in Wideband CDMA system and simple algorithm for Peak Windowing”, O. Väänänen, J. Vankka, K. Halonen, 2002 World Wireless Congress. This document proposes that the multicarrier transmission signal be attenuated when signal peaks occur, such that the signal is below a desired threshold value.